The recent rapidly inflating cost of many construction materials, notably steel pipe, has made it mandatory that an economical, sound, substantial and reliable method be found to install a casingless pile so that the rising cost of foundation work does not make the end cost of many projects prohibitive.
Hertofore, casingless concrete piles have usually been installed by two basic methods.
In the first method, a hole is drilled into the ground to a predetermined depth, using an earth auger. Then, depending upon the diameter of the hole, the shear strength of the soil and the gound water conditions, a casing or metal sleeve may or may not be inserted into the hole to prevent a soil cave-in and ground water infiltration. After this is done, the hole is filled with concrete. Then the casing may or may not be withdrawn as job conditions and specifications dictate.
Normally, piles or caissons are required because of unsuitable soil conditions, so usually a casing is required for each hole. This method is satisfactorily used for casisson 30 inches and larger in diameter where it is possible for a man to enter the caisson for cleanup work, and where the protected hole can be satisfactorily pumped.
However, in the smaller sizes, the system is uneconomical inasmuch as it takes approximately as long to dry auger a small-diameter caisson as it does a large-diameter one, providing drilling equipment is not underpowered. If the smaller hole cannot be satisfactorily pumped, then the pile must be poured by conventional tremie which is costly and time consuming. Also, under these conditions, it is virtually impossible to determine whether or not suitably bearing strata has been reached.
In the second method, a hollow-stemmed type of auger drill is screwed into the ground to a predetermined depth or torque, and a very fluid grout is pumped down the hollow stem to form the pile as the auger is withdrawn.
There are two basic variations employed in this second method and each has many inherent disadvantages, and often times neither method will produce a quality installation.
In the first of these basic methods, an attempt was made to maintain a seal at the periphery of the pile by not rotating the auger as it was withdrawn. This results in a very large force being needed to extract the auger and the entrapped soil from the hole, especially in granular materials. This is caused largely by the truncated cone of earth, often times extending from near the auger bit to a very large diameter circle of earth at the surface of the ground. Depending upon soil conditions, the base of the cone, for a 40 foot, 16 inch diameter pile, could be roughly 20 feet in diameter at the surface in granular material. In fact, it is this theory that prevails in using short-auger anchors to hold down trailers, etc., against hurricane forces in the Florida area.
In the second basic method used in installing the auger cast pile, the auger is turned as it is withdrawn from the hole and where ground water is present under pressure, often times the seal is lost and water may infiltrate into the grout and up along the sides of the pile. Piles of this nature have been uncovered where the pile was only partially set up and rivulets of sand and water extended vertically throughout the pile. These rivulets were caused by medium to high pressure ground water infiltration of the pile during the installation procedure.
Both of these basic concepts were derived during a time when auger cast piles were relatively short in length, that is 40 feet or less, and lightly loaded in the 20 to 40-ton range. Further, the cement grout being pumped down the hollow auger stem was not chosen because of its inherent acceptability, but because it was the only economical material that could be pumped down the small-diameter hollow auger stem that would not freeze up intransit through the stem.
Auger cast installations have been uncovered where the installed piles had a cemented outer shell approximately 1 inch thick, but where the inner core was still a sand, cement, and water slurry. In these cases, in order to maintain the flowability of the grout through the small-diameter hoses and stem, the water-cement ratio had been set so high that the material would not "set up." After the installation was made, the shell around the pile had "set up" because the surrounding soil had pulled enough water from the perimeter of the mix so that the mix could hydrate and become concrete.
Other methods have been devised. In Thornley et al. (supra) an expansible packing ring is provided around a base member through which concrete is pumped into the bottom of a hole. Burrell used an expansible mandrel through which concrete was pumped into the bottom of a shell. An expansible sleeve sealed between the mandrel and shell. This, however, was not a casingless pile.
None of the methods of installation presently being used for caseless piles can satisfy all of the parameters necessary to a good sound economical casingless pile installation.
These parameters can be defined, briefly, as follows:
1. Maintenance of the drilled diameter of the hole without the influx of water or soil cave-in during and after the drilling operation.
2. Assurance that the pile is seated into a good load bearing material.
3. Placement of high-strength, low slump concrete without segregation.
4. Prevention of the inter-mixing of concrete and soil during concrete placement.
5. Positive means of assuring continuous pressure in uninterrupted flow between the lower end of the concrete placement tube and the upper face of the concrete.
6. Speed and economy of installation.
It is the object of this invention to provide a method and means for casting piles in situ that is an improvement upon the present art and will satisfy the parameters outlined above.